Duplex radio transmission system



' Patented a.- 22,1929

UNITED STATES PATENT OFFICE FRANK CONRAD, 0F PITTSBURGH, PENNSYLVANIA,ASSIGNOR TO WESTINGHOUSE ELECTRIC d, MANUFACTURING COMPANY, ACORPORATION OF PENNSYLVANIA DUPLEX RADIO TRANSMISSION SYSTEH Applicationfiled ma 4,

simultaneous transmission of two messagesat substantially a singletransmission frequency.

Another object of my invention is to providea radio signalling system inwhich signals are transmitted simultaneously by modulation of theamplitude of oscillation and by modulation of the frequency ofoscillation.

It has been customary to transmit voice frequency signals by means ofmodulation devices, operating with oscillation generators,

which vary the amplitude of oscillation in accordance with the voicefrequency vibrations. With other systems it has been customary totransmit code signals by varying the frequency of oscillation inaccordance with the code signals. It has, however, been considered thatsuch systems were mutually exclusive and that a system embodying oneprinciple could not embody the other.

I have found that, at certain frequencies,

it becomes possible to operate both systems simultaneously from a singlehigh-frequency generator without interference therebetween. It ispossible to combine the two systems with an oscillation frequency ofabout 1500 kilocycles. I find that such operation is even moresatisfactory at frequencies above 3000 kilocycles. By this means, I amenabled to transmit two kinds of signals simultaneously from a singleoscillation generator without 4 interference therebetween.

Other objects and structural details of my invention will becomeapparent from the following description, when read in connection withthe accompanying drawing wherein the single figure is a diagrammaticrepresenta- 1925. semi m). 21,024. M

6 and the condenser 7. A second triode 9 has associated therewith, atransformer 11 and a telephonetransmitter 12. y

The anodes of the triodes 5 and 9 are connected in parallel to agenerator 14 through a choke coil or inductance 15. The cathodes of thetriodes 5 and!) are supplied by a gcnerator 16, the other terminal ofthe generator 14 being connected to the other side of the cathodecircuit.

A small variable reactor, such as a condenser 17, has one plate which isattached to the arm 18 of a relay 19. The coil of the relay 19 isconnected to a battery 21 and key 22. The plates of the condenser 17 areconnected by conductors 23 and 24 across a portion of the turns of theinductance 6.

The oscillating circuit of the generator is connected to the radiator bya conductor 25 and an inductance 26. a y

In the operation of my device, the amplitude of oscillations generatedby the generator triode 5 is controlled in any well-known way by theamount of current taken by'the modulator triode 9, which in turn iscontrolled by sound waves acting through the microphone 12 and thetransformer 11 upon the grid of the triode 9. Voice signals impingingupon the microphone 12 thus vary the amplitude ofthe oscillationsgenerated by the triode 5 and, therefore, the amplitude of theoscillations radiated from the aerial 1. These oscillations may then bereceived by a non-oscillating receiving system.

Simultaneously with the modulation of the amplitude of the oscillationsgenerated by the triode 5, the frequency of the said oscillations maybevaried by movements of one of the plates of the condenser 17, which hasthe effect of changing slightly the effective capacigenerator.

ing of the resonant circuits of the receiving apparatus. The variationof the condenser 17 and thereby the variation in frequency is controlledby movements of the key 22.

A second radio receiving apparatus may be rovided in which the receivingcircuit is iept in oscillation by an oscillator at a frequency differentfrom thefrequency of generation of the sending device. The usual heterodyne system may b'e-used for this pur- .pose. This frequency may bethe same as the frequency of generation of the sending apparatus whenthe sending key is up, thereby giving a zero beat frequency. Actuationof the key changes the condenser value and thus changes the frequency ofthe sending apparatus to a value which produces a beat frequency withinthe audible range. The listener will, therefore, hear a beat note whichcorresponds to the code signals formed by the key 22.

In simultaneous operation of the two signalling systems, the change infrequency necessary for the code signalling is insufiicient to produce aperceptible change at the ordinaryreceiving apparatus. Also, the carrierwave with which the local oscillator at the code receiving station isheterodyned is so much greater than the modulated components that thespeech modulations are substantially imperceptible and the heterodyneWave only is brought to theoperators attention.

By this means, I am enabled to provide a. system having a singlegenerator which is capable of simultaneous 'voice and code signalling atsubstantially a single frequency.

lVhile I have shown but one embodiment of my invention it is capable ofvarious modifications therefrom without departing from the spiritthereof. I desire, therefore, that my invention shall be limited only bythe appended claim and by the prior art.

I 'claim as my invention:

A duplex radio signalling system comprising a sending set and at leasttwo receiving sets, one of which is an ordinary receiving set, saidsending set including means to generate and radiate high-frequencyoscillations, means to modulate the amplitude thereof in accordance withone signal'and means to modulate the frequency thereof in accordancewith another signal, the degree of change of frequenc produced by saidlastmentioned modulatlon means being so small a percentage of said highfrequency that the set and will not interfere with the reception of thefirst-named signal by the ordinary receiving set.

In testimony whereof, I have hereunto subscribed my name this 20th dayofApril,

FRANK CONRAD.

